Air inlet systems for the compressors of turbines often include heating devices for raising the temperature of the air drawn into the compressor, as well as devices to attenuate noise. Heating tubes have been installed in the air inlet ducts of compressors to supply high temperature air extracted from the compressor for mixing with the ambient inlet air, thereby raising the temperature of the air supplied to the compressor. Baffles have also been employed in compressor air inlet ducts to reduce the noise generated by the air flowing through the inlet ducts to the compressor.
A prior system using compressor extraction air to heat the inlet air to the compressor employed a series of generally triangularly-shaped tubes disposed downstream of the silencer baffles in the compressor air inlet duct with each tube having an open apex facing in the upstream direction. Heated air extracted from the compressor flowed through a control valve choked to convert the high pressure air to low pressure air for supplying the low pressure heated air to the tubes. This heated air supply system included large piping, e.g., a large manifold disposed in the inlet air stream in the compressor inlet duct to conduct the heated low pressure air to the tubes. The low pressure heated air was supplied to the ambient air flowing in the compressor inlet duct through open apices of the tubes, which apices faced in an upstream direction. The heated air blown upstream was immediately turned downstream for flow back across the tubes. The reversed heated air and the inlet air formed vortices off the back edges of the tubes which aided in mixing the heated air and the ambient inlet air.
However, it was found that the temperature distribution of the air flowing to the compressor in that system was not uniform across the inlet duct. Icing on the inlet duct and compressor inlet remained a problem, particularly when operating at reduced inlet guide vane angles required to extend the range of dry low NO.sub.x premixed operation. Also, substantial pressure loss occurred because of the large ducting required in the air flow stream to supply the heated low pressure air to the inlet air stream. More specifically, this prior system oriented the multiple triangularly-shaped tubes in a direction generally perpendicular to the planes of the silencer baffles and which arrangement contributed to the lack of uniform temperature distribution across the inlet air duct. It will be appreciated that high inlet temperature distribution reduces turbine efficiency, reduces compressor pressure ratio operating margin and may physically damage compressor parts. Also, it was found, in such prior low pressure system, that the heated air supplied to the tubes generally did not have a uniform temperature distribution along the length of the tubes. Consequently, it has been determined that an improved inlet bleed heat system is necessary to reduce the pressure loss across the inlet, prevent ice build-up on the inlet ducting and compressor inlet, provide a uniform compressor inlet temperature profile, and to protect the turbine compressor from icing when operated at reduced inlet guide vane angles required to extend the range of operation in the dry low No.sub.x premixed mode.